A reticulocyte is the youngest red blood cell that has been denucleated. An increase in reticulocyte serves as an index of the increase in red blood cell production in bone marrow. The inside of a reticulocyte contains RNA that is a residue of a nucleus, which is present as a reticular granule. While it is observed as a large polychromatic red blood cell in an ordinary Wright's staining, it may be stained as a reticule or granule in a supravital staining. It is represented usually as a reticulocyte rate per 1000 red blood cells (0/00; pro Mille).
The reticulocyte count increases or decreases in response to increase or decrease in the production of red blood cells. Since its elevation precedes the red blood cell count or hemoglobin level in the treatment of an iron deficiency anemia or megaloblastic anemia, it is useful in evaluating therapeutic effect at an early stage.
Diseases characterized by an increase in reticulocyte count include, for example, hemolytic anemia, vitamin B12 deficiency anemia, folic acid deficiency anemia, iron deficiency anemia, megaloblastic anemia, and the like. Hemolytic anemia is accompanied by a reduced lifetime of red blood cells, increased serum, indirect reacting bilirubin, reduced haptoglobin, and the like. Diseases exhibiting reduced reticulocyte count should be diagnosed based on bone marrow findings in addition to peripheral blood findings, and include, for example, acute leukemia, aplastic anemia, and the like.
With regard to reticulocyte counting technology, a flow cytometric counting technology has been employed (see, for example: U.S. Pat. Nos. 4,544,546; 4,957,870; 5,360,739; and 5,821.127), and an automatic blood cell counter incorporating this technology has been marketed.
An automatic blood cell counting device may measure the degree of maturity of a reticulocyte in addition to reticulocyte count (rate). The clinical significance of measuring the degree of maturity of a reticulocyte is related to the chemotherapy, hematopoietic recovery monitoring after a bone marrow implantation. When attempts are made to recover bone marrow function that was once suppressed (e.g., by an anticancer agent) by discontinuing chemotherapy, an increase in the count of rather young reticulocytes reportedly precedes others by 2 to 3 days (see, for example: “High fluorescence reticulocytes are an indicator of bone marrow recovery after chemotherapy” in Eur. J. Haematol., 1995, 54, 61-63). Since clinical practitioners expect bone marrow function recovery to be achieved as soon as possible, an identification of the recovery at an early stage is of a great significance.
In the past, the count and the degree of maturity of a reticulocyte were typically the only parameters taken into consideration in diagnosing an anemia or in monitoring the hematopoietic condition after chemotherapy. Heretofore, the size of a newly produced red blood cell (reticulocyte) has not been taken into consideration.
It would be extremely useful in investigating the effect of a therapeutic agent to monitor the size of a reticulocyte. Nevertheless, conventional approaches, which take only the number (rate) or maturity of reticulocytes into consideration, are at risk of producing false judgments as to whether a newly produced red blood cell has a normal function or not.
Moreover, conventional measurements of reticulocytes, when employed alone, pose a problem in that a red blood cell having a low hemoglobin content can not be detected even if it is produced. In erythropoietin therapy for a renal anemia patient in which the production of red blood cells in the bone marrow is increased, a simple deficiency of stored iron is combined with the inability to transport the stored iron to the bone marrow on time (i.e., functional iron deficiency). This is observed clinically when the response to erythropoietin becomes poor in spite of the normal index of kinetics for existing iron species (e.g., iron and ferritin), and leads to the production of a hypochromic red blood cell whose hemoglobin level is reduced.
As described above, the conditions of functional iron deficiency can not be fully assessed if only the number and the rate of a reticulocyte are observed.